Responding to ever increasing demands for an optical storage system that can store a large amount of data, such as data for a motion picture film, various types of holographic digital data storage systems incorporating therein a holographic medium made of a photo-refractive crystal such as lithium niobate or the like have been recently developed for realizing high density optical storage capabilities.
The holographic digital data storage system allows a modulated signal beam having information therein to coherently interfere with a reference beam to generate an interference pattern therebetween and, then, controls the interference pattern to be stored as index perturbations (holograms) in a specific recording location of the holographic medium such as a photo-refractive crystal, wherein the photo-refractive crystal is a material which may react differently on interference patterns depending on the respective amplitudes and phases thereof.
To realize high-density storage capabilities, many schemes for hologram multiplexing have been suggested, such as angular multiplexing, wavelength multiplexing, shift multiplexing and phase code multiplexing. Recently, a correlation multiplexing has received considerable attention for its sharp spatial shift selectivity, wherein the correlation multiplexing employs a random pattern (RP) referencing scheme, a speckle pattern referencing scheme or a complex referencing scheme in which a quasi-random-phased speckle wave front is used as a reference beam. Large numbers of holograms may therefore be multiplexed in essentially a same volume of the holographic medium through only a micron-size spatial translation of the holographic medium relative to the reference beam.
Referring to FIG. 1, there is shown a block diagram for illustrating a conventional holographic digital data storage system multiplexed by using a correlation multiplexing. The conventional holographic digital data storage system includes a laser 100, a beam splitter 101, a first and a second mirror 102 and 104, a spatial light modulator (SLM) 105, a diffuser 108, a holographic medium 110, a shutter 111, a linear stage 112 and a charge coupled device (CCD) 120.
In a storing mode, a coherent monochromatic beam, e.g., a laser beam emitted from the laser 100, impinges onto the beam splitter 101. The beam splitter 101 splits the laser beam into a reference beam R and a signal beam S. The reference beam R is a portion of the laser beam transmitted through the beam splitter 101 and the signal beam S is a remaining portion of the laser beam reflected from the beam splitter 101. After being reflected by the first mirror 102, the reference beam enters into the diffuser 108. The diffuser 108 transforms the reference beam into a complex reference beam RD for a correlation multiplexing.
In the meantime, the signal beam S is reflected by the second mirror 104 and, then, enters into the SLM 105. Since a sequence of digital page data is sequentially provided to the SLM 105, the signal beam S is sequentially modulated with the digital page data to generate a modulated signal beam SM.
The modulated signal beam SM and the complex reference beam RD are converged on the holographic medium 110 to generate a sequence of interference patterns to be sequentially stored in the holographic medium 110.
To read out the stored data, a retrieving reference beam with characteristics matching with those of the reference beam used during the storing mode must be illuminated precisely to a specific storing location of the holographic medium and diffracts off the stored index perturbations to reconstruct a reconstructed signal beam corresponding to the modulated signal beam.
Specifically, in a retrieving mode, the shutter 111 located along a path of the signal beam turns to be closed so that only a retrieving reference beam R may be obtained from the coherent monochromatic beam, wherein the retrieving reference beam R of the retrieving mode is substantially same as the reference beam R of the storing mode.
After being reflected by the first mirror 102, the retrieving reference beam enters into the diffuser 108. The diffuser 108 transforms the retrieving reference beam into a complex retrieving reference beam RD. Therefore, the complex retrieving reference beam is substantially same as the complex reference beam in the storing mode.
The complex retrieving reference beams RD is illuminated on the holographic medium 110 in which the interference patterns have been sequentially stored, to sequentially reconstruct a reconstructed signal beam. The reconstructed signal beam is substantially a diffracted beam which is generated from the interference patterns through irradiation of the complex retrieving reference beams RD into the holographic medium 110. The reconstructed signal beam is captured with a predetermined interval to sequentially recover the digital page data.
Usually, a high-precise linear stage on which the holographic medium is installed has been precisely controlled with the predetermined interval by a DC servo motor, to determine the specific storing location. In other words, after the storing location is detected by using the DC servo motor, the CCD camera may have captured the reconstructed signal beam to read out the digital page data. Since, therefore, the DC servo motor is controlled to sequentially move the high-precise linear stage by a predetermined interval/distance, the storing locations of the digital page data are not precisely detected.